SIGMA PEG (Sigma Pegasi). About 10 degrees northwest of the Circlet of Pisces is a pair of stars a degree apart,
fifth magnitude Rho (4.90) then Sigma
(5.16) Pegasi (in Pegasus, the Flying
Horse of the Andromeda myth), that
point also northwesterly toward fourth magnitude Xi Peg. Rho is a
class A1 dwarf 312 light years
away. Much closer, at a distance of 89.0 light years (with an
uncertainty of just 0.7), Sigma is a class F (F7) subgiant more like the Sun. It also carries a special treasure, a faint
companion called Sigma B, rendering bright Sigma itself "Sigma A,"
which is usually known by its Flamsteed number, 49 Pegasi.
Sigma Peg proper has been subjected to a two dozen temperature measures
that average to a pretty precise 6245 Kelvin. With little but visual
radiation to deal with, the distance gives a luminosity of 5.15 times
that of the Sun, which in turn yields a radius 1.94 times solar. Cooler
than the "rotation break" around F2 (above which stars rotate freely),
Sigma is a leisurely rotator, with a projected equatorial speed of 6
kilometers per second (triple the solar value), allowing a rotation
period of up to 17.4 days. Application of theory gives a mass of 1.28
Suns and shows that the star, if not a subgiant (a star whose core
hydrogen fuel has run out) is pretty close to being one after a lifetime
of just over four billion years. The age explains the slow rotation,
as magnetic effects act as a brake, the star consistently no longer
showing much, if any, magnetic activity. At a separation of 250
seconds of arc lies fourteenth magnitude Sigma B, a class M4 red dwarf.
You'd pay little attention to it (red dwarfs are everywhere) except
that it's keeping pace with Sigma A and seems quite to belong to the brighter star. (Be
careful of the name. Possible faint companions of 10th and 16th
magnitude respectively 16 and 15 seconds away, which may be just in
the line of sight, have been called "B" and "C," rendering the red dwarf
"Sigma D." We'll stick with "B.") Such companionship is not uncommon;
after all, the nearest bright star, Alpha
Centauri, is followed by the red dwarf Proxima Cen (which is slightly closer us).
However Sigma Peg B has another surprise in store. Close examination
reveals that it too is double, made of two red dwarfs just a couple
tenths of a second of arc apart (reminiscent of Castor C). Separated by at least 8
Astronomical Units, the two take more than 30 years to orbit each other.
Given a physical distance of 6800 AU, the orbital period of the twin
red dwarfs around Sigma proper must be at least 415,000 years. It's
remarkable that with an age some four billion years that the
companionship has survived. If indeed it has, as the two may actually
be drifting apart. Of equal interest is that Sigma is speeding along
at 68 kilometers per second relative to the Sun, roughly 4.5 times
average, suggesting that it's a visitor to the local community from
a different part of the Galaxy.
Consistently, Sigma's iron-to-hydrogen ratio is low, 60 percent solar,
and the red dwarfs are as bit faint for the class, shifting them a
somewhat toward the realm of subdwarfs, though any such conclusions
are at best made on minimal data.
Written byJim Kaler 11/06/15. Return to STARS.